The present invention relates generally to electrical antennas and in particular to an antenna generating an efficient and uniform electromagnetic field for plasma generation and the like.
High frequency electrical fields for the generation of plasma may make use of a conductive coil (“field applicator”) driven by an AC current oscillating in the megahertz to gigahertz range. A gas within the coil receives energy from the coil through inductive coupling exciting the gas into a plasma state.
Such inductive coupling techniques for generating plasma have a number of significant problems. First, normally the conductive coil must have multiple “turns” and each turn exhibits a mutual capacitance with adjacent turns of the loop creating field (and hence plasma) manifest as of nonuniform plasma ion speeds, trajectories and densities. Non-uniformities in the plasma may adversely affect applications were uniform plasma is required (for example, for etching in the integrated circuit industry) and may waste energy on undesired plasma processes. The mutual capacitance also limits the voltage that may be applied to the conductive coil without dielectric breakdown between the turns of the coil.
Second, the large amount of electrical power and hence large amounts of electrical current required to pass through the conductive coil produce significant resistive heating requiring complicated or bulky cooling structures. The use of highly conductive materials, such as copper, can reduce resistive losses, but the use of copper and similar metals is complicated by the susceptibility of such highly conductive materials to corrosion and melting in the harsh environment of the plasma.
Third, efficient driving of the conductive loop requires that the loop be part of a resonant structure implemented by placing a tuning capacitor into the coil circuit. Capacitors suitable for this purpose are expensive and bulky.